Vehicle and transportation sector-related emissions continue to be a leading source of greenhouse gas (GHG) and air pollution in urban areas around the globe. As an example, there were over 260 million vehicles in the United States (U.S.) in 2012 that emitted 33% (1,750 million metric tons) of total U.S. carbon dioxide (CO2) emissions. In the same year, the U.S. transportation sector share of total U.S. emissions for carbon monoxide (CO), nitrogen oxides (NOx) and PM were 54%, 59%, and 8%, respectively. Therefore, significant resources continue to be focused on emission reduction tactics which typically fall into two categories: current fleet inventory upgrade (e.g., roadside and/or engine bay inspection and maintenance (I/M) programs, aftermarket engine/vehicle/fuel programs, etc.) and new vehicle manufacturing (e.g., revisions of emissions control standards for newly manufactured vehicles, etc.).
The U.S. Environmental Protection Agency (USEPA) defines airborne particulate as “a complex mixture of extremely small particles and liquid droplets . . . made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles.” In recent decades, the USEPA, like many other environmental regulatory agencies, has invested significant time, effort, and resources in a wide range of particulate monitoring activities. Early vehicle exhaust particulate monitoring programs employed relatively crude gravimetric methods such as capturing particulate emissions samples on pre-cleaned and weighed filter papers and re-weighing these post-sampling to determine PM mass by difference.
Despite the highly labor-intensive nature of these methods, refinements of these early gravimetric methods (typically employing more inert filter substrates) are still widely used even today. However, significant time and effort has also been focused on the development of other particulate monitoring techniques, most notably methods that could be automated to increase quantitative accuracy, increase sampling efficiency, reduce sampling errors, and reduce associated costs. In more recent years these automated methods have become the basis of more recent rounds of environmental particulate regulation.
The earliest automated particulate measurement technologies were based on measurement principles that provided mass measures or measures that could be readily calibrated against mass measures. Two examples are tapered element oscillation microbalance (TEOM) methods used for routine automatic monitoring of adherence to ambient PM air quality standards and opacity-based methods used in vehicle exhaust PM emissions TIM inspection testing procedures.
This early focus on PM may have reflected both a design to align these newer methods with more established gravimetric testing procedures and an acknowledgement that these measurement technologies were likely the most convenient and accurate options at the time. However, as particulate sources became cleaner in response to PM-focused regulations and associated consumer demand, investigators observed that emission source particulate size distributions tended to become finer and that the number of ultra-fine particles in ambient air is more closely correlated with health effects than the total mass of those particles.
More recently, the development of a PN measurement system by the European Union in 2007 was to enable a more accurate and repeatable identification process of particulates (individual emitted particles). Additional EU objectives were to minimize required changes to the current type approval facilities such as laboratories, to employ an understandable metric, and for the system to be simple to operate. The EU PN system was developed to avoid the possible requirement for correction factors, an issue that has hampered the development of similar measurement initiatives in the U.S.
Accurate emission(s) data are required for a wide range of vehicle and point-source related applications in order to properly evaluate the impact of emission reduction strategies. However, given the complex and (perhaps more importantly) evolving nature of particulate emissions, a single PM or PN metric is unlikely to provide a robust “catch-all” metric for emission-reduction activities. Therefore, it is important to differentiate between the differing sizes of PM/PN in order to both better understand the process that produced them and aid in the identification of potential solution(s) for their reduction. Atmospheric particulate sizes typically range from a few nanometers to tens of micrometers in diameter. The coarsest material (typically 10 micrometers and larger) is predominantly from biological sources (e.g., spores, pollen, bacteria, etc.) and/or mineral sources (e.g., land erosion, construction work, etc.). Finer particles (less than a micrometer) are typically formed by nucleation, condensation, and agglomeration processes, such as a result of atmospheric chemistry and combustion processes.
PM/PN that originates from combustion processes are typically of interest.
Particles smaller than 0.1 micrometer (“ultra-fines”) formed as the result of fuel combustion/exhaust emissions processes are associated primarily with internal combustion engines, such as those in on-road, off-road, and non-road vehicle activities. Such particulates have been cited as dangerous due to toxic trace compounds (e.g., heavy metals, polycyclic aromatic hydrocarbons, etc.) often contained in the particulates. The USEPA and the European Union's Joint Research Centre (JRC) have both declared that the concentration of such particles is highly variable, and appears to demonstrate a significant pattern of variation, especially close to urban areas and traffic congestion.
Traditionally, vehicle testing is often performed in a laboratory with a chassis or engine dynamometer, following government-approved testing cycles (e.g., Euro IV, U.S. FTP, or other global standards). However, these standard test procedures, much like the traditional gravimetric PM measurement procedures, are no longer representative of their real-world counterparts and the growing gap between the emission reductions and fuel savings routinely achieved by modern cars on these test cycles and their on-road (or “off cycle”) performance has been widely reported.
Investigators already widely acknowledge the “off-cycle” gap to be of the order of 40-60% for fuel consumption and CO2 emissions, and up to 400% (4 times approval levels) for NOx emissions. Although the challenges associated with real world measurement of vehicle PM/PN emissions using current technology significantly hinder the direct measurement of similar trends for particulates, indirect measurement methods, most notably tunnel studies, indicate that these also may be orders of magnitude higher than lab-based measurements.
Concerns about these “off-cycle” emissions have led to an increased demand for real-world vehicle emission data and for new regulations based on real-world vehicle performance.
Portable Emissions Measurement Systems (PEMS) are vehicle monitoring platforms that can be temporarily attached to a target vehicle to provide a direct measure of vehicle emissions as the vehicle is used in actual service. For example, the “Real-time On-road Vehicle Emissions Reporter” (ROVER) disclosed by the USEPA is an on-board testing system temporarily mounted on a vehicle for the purposes of measuring real-world emissions while the vehicle is driven on the roadway. Commercial systems, many based on the original ROVER concept, are now used to collect real-world vehicle emission data.
However, the disadvantages of the conventional PEMS approach are also significant. Due to the fact that all commercially available PEMS equipment requires that the sample be transported well away from the exhaust stack or tailpipe, issues such as heat changes, sample degradation, power requirements, and system complexity typically introduce accuracy and dependability problems. These issues significantly impact the investigation of emission reduction tactics, as PEMS are limited by their design, size, and weight, and thereby limit the amount of real-world vehicles and test data that can be collected in a “true” real-world scenario. Simply put, PEMS-related activities (equipment installation, maintenance, upkeep, charging, filter replacements, supervised operation, uninstall, etc.) often hinder the ability to collect truly representative “real-world” data in sufficient volumes and/or at reasonable costs.
Present vehicle exhaust PM/PN PEMS have significant power demands attributable to the sensor design, sample dilution, and flow measurement(s). Due to these challenges, currently-available PEMS do not have the capability to run self-powered for more than a few hours or, in some cases, even more than a few minutes, which significantly restricts sampling options. Also, the accompanying weight of such systems demands that the device(s) be mounted well away from the exhaust outlet, further limiting the target vehicle's performance under typical driving conditions. In addition, the associated use of long sample line(s) required to transport the sample to the sensors introduces a range of sample integrity issues that particulates are particularly sensitive to.
One problem is that increased power to heat or condition the sample while it is being transferred from the exhaust to the sensor adds significantly to the power requirements and weight of the PEMS unit. Additionally, the increased length of the sample line(s) increases the surface area for interaction with the sample (e.g., water vapor condensation and particulate deposition), which, in turn, must be accounted for and expelled/corrected.
There are several problems with increased length of sample tubes and/or lines that PM/PN measurements are particularly sensitive to.
First, the additional length means an increase in the power required to pump a sample at a given flow rate due to flow friction, which increases with the length of the sample tubing, and necessitates a more powerful pump and a need for a larger, more powerful battery.
Second, in addition to the direct increase to the size and weight of the testing device, longer sample lines introduce additional weight, bulk, and complexity to the testing process. They must be properly and safely clamped, secured, or tied along the entire run of sample line. This increases the chance of safety issues, such as improperly secured lines that get caught up in running machinery and moving parts, etc. Length negatively affects the reliability of the system. The increased length of the sample line(s) also increases the surface area for particulate deposition, which, in turn, must be accounted for and corrected.
Third, a longer length of sample tubing requires a proportionally increased amount of insulation and/or supplied heat to prevent the condensation of liquid water and deterioration of the sample as it travels to the testing device and cools. Most exhaust gas samples contain significant amounts of water. If this is allowed to condense during sample transfer or analysis, the resulting liquid water can interact with and degrade some pollutants, most notably the more reactive gaseous species and particulates. However, the increased insulation adds significant weight and bulk to the sample lines, and in many cases PEMS units require additional dedicated power supplies for the sample line(s) heaters.
Fourth, as sample line length increases, it also increases the amount of setup/teardown time required to perform testing. The additional time required to properly install long sample lines directly impacts the number of accurate and safe tests that can be completed in a shift.
Finally, another problem with present vehicle PM/PN PEMS is their focus on a single sensing method. Although these current PM/PN systems have been accepted as accurate by various federal, state, local, and global evaluation standards, a single measurement method-based solution is typically advocated by regulatory agencies. Extremely accurate sensing of one variable may require equipment of large size and weight. It also means that associated reliability of any analyte measurement is intrinsically linked to one measurement principle and, therefore, requires the continued representativeness of the associated metric.
Each sensing technique uses a different approach and has a different bias with the PM/PN that is being sensed and recorded. Unlike gaseous pollutants, such as CO2 or NOx, PM/PN is not one chemical species. The exact constitution of PM/PN emissions includes complex structures, for example a solid phase carbon particle with liquid phase hydrocarbons adsorbed onto its surface, and both of these phases can incorporate, adsorb, or absorb numerous species in numerous distributions. Furthermore, PM/PN exists in a wide range of sizes, and health concerns have been associated with PM/PN of aerodynamic diameter from 10 micrometers to less than 100 nanometers. Any one measurement technique will provide results that are biased by the type of PM/PN the measurement technique is most sensitive to and no one measurement technique can be sensitive to the complete range of PM/PN chemical and physical structures. Thus, PM/PN, by its very nature, cannot be fully characterized by any one sensing technique, however accurate it may be.
This point is illustrated by considering the existing California Heavy-Duty I/M test procedure. This incorporates a measurement based on opacity (a measure of light extinction). When this was first introduced, it was a highly effective test because it provided a good measure of the larger coarser material that then represented a significant fraction of exhaust PM mass. More recent improvements to vehicle engine management systems and exhaust emissions abatement systems have both reduced the amount of particulate emitted by vehicles and the size ranges it is typically emitted in. The opacity method is not sensitive to the smaller amounts of finer material that modern vehicle typically produce. As a result, a faulty modern vehicle can emit large amounts of particulates, often well above regulatory limits, but still pass an TIM test because the emitted particulate is too fine to be detected using opacity.
What is needed is an improved PM and PN measurement device that is both easier to deploy (e.g., smaller, lighter weight, lower energy demand) and provides an ability to provide both a measure of PM/PN on the basis of current standards and also identify, characterize, or map onto the changing properties of particulates as their emission sources change.